Magnetically-enhanced cathode sputtering devices have been used extensively to deposit a thin layer of a target material on a substrate. One common application for such devices is in the manufacture of compact discs (CDs) having a thin layer of aluminum target material along a surface of the polycarbonate substrate disc. Typically, the substrate is located in the presence of a vacuum (10.sup.-3 Torr) and a small concentration of Argon gas is introduced. The target is energized with a cathode voltage which ionizes the argon and generates a plasma. The target material is atomized by the plasma and is "sputtered" toward the disc, which is adjacent a grounded anode. A basic design for a magnetron sputtering device is disclosed in U.S. Pat. No. 4,166,018. Various improvements on this basic concept have also occurred.
FIG. 1 schematically illustrates a magnetron sputtering device according to the prior art. The device 20 comprises a magnet assembly 22 located adjacent a target 24 along a back surface of the target 26 opposite a substrate 28 to be coated. The magnet assembly 22, according to this example, comprises permanent magnets mounted on a plate of magnetically-permeable material 32. A pair of outer magnet rows 30 surround an inner magnet row 34. The outer rows 30 are generally joined together at their ends by curved sections (not shown) of magnetic material to form a closed loop around the inner row 34. The outer rows 30 are oriented so that their north poles (N) are adjacent the back surface 26 of the target 24. Similarly, the inner or center row 34 is oriented so that its south pole (S) is most closely adjacent the target's back surface 26. The north-to-south axis of each magnet is, likewise, aligned substantially perpendicularly to the back surface 26 of the target 24.
The location of the magnet rows 30 and 34 and the orientation of their poles N and S generate a magnetic field defined by a series of magnetic flux lines 36 that penetrate the target 24 and that form a closed loop, tunnel-like, arched section 38 over the front or "sputtering" surface 40 of the target. The midsection 42 of the arch 36 is approximately adjacent the sputtering surface 40 and defines flux lines that are substantially parallel to the plane of the sputtering surface 40.
The flat arched section 42 confines the charged plasma in close proximity to the sputtering surface 40. This confinement facilitates the sputtering or projection of material onto a desired portion of the substrate 28. Similarly, the arched section 38 of the magnetic field prevents spreading of the plasma along the target's sputtering surface 40 and maintains the plasma laterally within the area of the arch. Thus, the transfer of sputtered material occurs within a well-defined region of the substrate 28 based upon the shape of the arch.
A disadvantage of sputtering according to FIG. 1 is that the sides 46, 48, 50 and 52 of the magnetic field arch 36 concentrate erosion of sputtering target 24 toward the center of the arch 38 and result in the formation of erosion trenches 54 and 56 in the target that define an approximately V-shaped cross section. Hence, a large portion of target material adjacent the target's center and outer ends remains unused. The uneven erosion necessitates replacement of targets at shorter time intervals than if more of the target were actually sputtered.
The use of thicker targets in the process of FIG. 1 is not generally effective. The thickness of the target is limited by the strength of the magnets. The arch 38 becomes too distant from the magnets and thus the field becomes too weak to properly contain the plasma.
An alternate configuration for a magnetron sputtering apparatus is disclosed in U.S. Pat. No. 4,486,287. This configuration is shown generally in FIG. 2 and can be used for coating of circular CD surfaces. The magnet assembly 51 comprises a pair of concentric inner and outer annular magnets 55 and 57, respectively, centered about an axis 59. The magnets are aligned so that their north and south poles, N and S, respectively, are oriented along an alignment that is parallel to the plane of the target 60 and the plane of the substrate 61.
The target 60 in this example comprises an annular ring of sputtering material having a sputtering surface 62 located adjacent the inner and outer magnets 55 and 57. The magnets 55 and 57 generate a magnetic field defining a plurality of substantially parallel flux lines 64 that are located in front of and behind the sputtering surface of the target 60. The target 60 also includes a pair of concentric confining walls 66 and 68 at the inner and outer edges of the target that enable plasma generated from the sputtering surface 62 to be confined within the area of the flux lines 64 directly adjacent the sputtering surface 62.
The configuration of FIG. 2 enables efficient utilization of the target and facilitates sputtering over the entire target sputtering surface 62. This configuration also enables the use of thicker targets for prolonged sputtering without target replacement. However, orientation of the inner magnet 55 poles N and S parallel to the substrate plane necessitates a relatively large diameter for the inner magnet assembly. The enlarged diameter of the inner magnet reduces the area of the substrate center that can be sputtered effectively. Thus, the magnet configuration of FIG. 2 is sometimes unacceptable when an area close to the center of the substrate 61 must be sputtered.
In view of the disadvantages of the prior art, it is an object of this invention to provide a magnetron sputtering cathode assembly having an increased target life with an improved target erosion pattern. The cathode assembly should be able to accept relatively thick targets and should enable long term use without target replacement or readjustment. The cathode should be able to sputter magnetic materials and nonmagnetic materials and should enable sputtering of a large area of the substrate.